This invention relates in general to tracer or duplicating lathes and in particular to a dual template assembly for permitting rough and finish cuts to be made in a workpiece by successive passes of a cutting tool of a lathe.
The lathe is one of the most basic of all of the metal and wood working tools. Lathes are designed to support a workpiece between centers or in a chuck and to rotate the workpiece against a non-rotating cutting tool. As a result, material is selectively removed from the workpiece in order to form the outer surface thereof into a desired shape or profile. Cylindrical and conical parts, such as pins, bolts, shafts, discs, and the like, are often formed on lathes.
In particular, lathes are well suited for repeatedly duplicating the shape of a workpiece. In order to accomplish this, the lathe is usually equipped with a tracing mechanism. The tracing mechanism includes a flat planar template having an edge surface which is formed in the same shape as the desired profile for the workpiece. A tracing stylus is positioned against the edge surface of the template and moved therealong such that the tracing stylus follows the edge surface. A cutting tool is connected to the tracing stylus for movement therewith. The tracing stylus and the cutting tool are usually moved co-axially with the axis of rotation of the workpiece. Thus, the cutting tool cuts the pattern of the edge surface of the template into the workpiece as it is moved axially therealong. So long as the template is securely attached to the lathe and the tracing stylus is securely connected to the cutting tool, the tracer lathe can quickly reproduce the shape of a workpiece.
Although the tracer lathe is useful for repeatedly duplicating the general shape of a workpiece, it has not been entirely successful in providing a high degree of precision in such reproduced workpieces. This is because the workpiece, prior to being machined on the lathe, is usually initially formed having a shape which only very roughly approximates the desired final shape. For example, the workpiece may be initially formed having a diameter which is approximately two hundred thousandths of an inch larger than the desired finished diameter. Many tracer lathes permit only a single pass of the tracing stylus along the single template. Accordingly, the cutting tool is required to remove a relatively large amount of material from the workpiece (one hundred thousandths of an inch along the radius thereof) on the single pass of the cutting tool across the workpiece. Since the cutting tool is required to remove such a large amount of material, the accuracy of such removal is reduced, even when the speed at which the cutting tool is moved along the workpiece is reduced. In other words, the tracer lathe cannot form the workpiece to highly accurate finish tolerances, on the order of two thousandths of an inch, because such finish tolerances are achievable only when a smaller amount of material is removed from the workpiece.
In order to increase the accuracy of the material removing process, some tracer lathes are provided with a tracing stylus which is selectively movable relative to the cutting tool The tracing stylus is initially moved to a first tracing position relative to the cutting tool for engagement with the template during a first pass of the cutting tool along the workpiece. This first pass causes a rough cut to be made in the workpiece which closely approximates the final desired shape. After the rough cut pass is completed, the tracing stylus is moved to a second tracing position relative to the cutting tool. Then, the tracing stylus is again moved along the template such that the cutting tool removes additional metal from the workpiece. This second pass of the cutting tool causes a finish cut to be made in the workpiece, thereby achieving the final desired shape. Although this type of tracer lathe provides improved accuracy because less material is removed from the workpiece on the final pass of the cutting tool, the mechanism which supports and moves the tracing stylus relative to the cutting tool undesirably introduces additional margins of error which limit the accuracy thereof. Also, such mechanisms are usually complicated and expensive.
Some lathes are operated by a computer in accordance with a predetermined program which defines the movements of the cutting tool relative to the workpiece. Computer numerical control (commonly referred to as CNC) lathes of this type have been provided which make successive rough and finish cutting passes along a workpiece. Since the movement of the cutting tool is controlled by a computer, however, CNC lathes do not use a template or a tracing stylus. Although CNC lathes provide a high degree of accuracy, they are very expensive in comparison with conventional tracer lathes.